This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to German Application No. P 199 01 282.2 filed on Jan. 15, 1999, the entire content of which is incorporated herein by reference.
The present invention generally relates to linear bearings. More particularly, the present invention pertains to a rolling bearing cage for a linear bearing and an injection molding tool for producing a cage for a linear bearing.
Injection molding tools for producing linear bearing cages are known. A rolling bearing cage made of an injection-moldable material is described in German Offenlegungsschrift 36 35 261 and is made in one piece of an endless strip injection-molded in sections in a tool. The recesses for receiving rolling bodies are provided with protrusions extending out of the base element of the cage and extend around the balls inserted into the cage to prevent the balls from falling out of the cage.
A similar version of a rolling bearing cage is known from DE 37 22 651 A1. Here, the base element of the cage is also provided with recesses which are intended to prevent the inserted balls from falling out.
One type of known linear bearing cage construction is illustrated in FIG. 1. The illustrated cross roller cage halves 11xe2x80x2 are provided with openings for receiving rollers, with pairs of adjacent or successive rollers being offset by 90xc2x0 with respect to each other. The cage 2xe2x80x2 is shown in the disassembled state in FIG. 1 and in the assembled state in FIG. 2. The two parts 11xe2x80x2 of the cage that are visible in FIG. 1 are connected, and for this purpose appropriate clip pins are provided. The receiver pockets 10xe2x80x2 for the rolling elements as well as the holding protrusions 12xe2x80x2 are thus formed, thereby preventing the rolling elements from falling out of the cage.
Another known version of a cage is illustrated in FIGS. 3 and 4. In this version, individual receiver elements 11xe2x80x2 for individual rollers are injection-molded. A plurality of these individual elements 11xe2x80x2 which are shown in FIG. 3 are clipped together to create the entire cage arrangement which is depicted in FIG. 4. The individual cage elements also have holding protrusions 12xe2x80x2 which prevent the rolling body from falling out of the cage element.
On the one hand, the above mentioned cages are frequently needed in a very long one-piece version. On the other hand, the cages are also needed for quite small and numerous balls for increasing the support. It has been found that problems can arise when injection molding the cage.
The thermoplastic material injected into an injection molding tool must fill the entire mold cavity. This can cause problems, particularly in connection with cages for small balls, because the plastic material cools too rapidly and xe2x80x9cfreezesxe2x80x9d. Thus, the protrusions needed for holding the balls are not shaped with sufficient accuracy. Easy-flowing plastic materials, for example LCPs (liquid crystal polymer plastics), are as a general rule very expensive and typically do not have the required elasticity to be used in this context.
Furthermore, multiple cross roller cages are also known. However these cages must be connected by an additional assembly process. This means higher production costs because of the assembly tools, as well as specially formed snap-together elements on the cage. The snap-together elements also increase the division between the rolling bodies and therefore reduce the support capability.
Sheet metal cages are also known which typically do not have these same problems in connection with their production. However, the disadvantage of metal cages is that the production costs associated with manufacturing metal cages are relatively high, they display relatively low elasticity and typically provide a ball guidance that is not as good as a plastic cage.
Known injection molding tools for producing cages for linear bearings have the further disadvantage that it is frequently necessary to accomplish the shaping of the holding protrusions for the rolling elements by way of complicated injection molding tools designed with movable slider elements. Although it is possible by way of these tools to relatively precisely produce the desired shape of the holding protrusions as well as the stop faces of the rolling bodies, the required injection molding tool is quite complicated and therefore rather expensive.
In light of the foregoing, a need exists for an injection molding tool that is able to produce a relatively exact shaping of the holding projections as well as the stop faces for the rolling bodies.
A need also exists for an injection molding tool having a design that is more simple than other known injection molding tools to thereby produce cages in a more cost effective manner.
It would also be desirable to produce rolling bearing cages for use in linear bearings in such a way that relatively long cages, particularly suited for receiving very small balls or respectively rollers, can be reliably and reproducibly made, without the problems associated with other known injection molding techniques.
One aspect of the invention involves an injection molding tool for producing a one-piece cage for a linear bearing in which several rolling elements are positioned in the one-piece cage in a direction along a longitudinal extent of the one-piece cage. The injection molding tool includes first and second tool halves between which is defined a cavity of the tool which receives injection molding material during injection molding of the one-piece cage. The first tool half is provided with a plurality of first protrusions fixedly connected with the first tool half and extending into the cavity of the tool during the injection molding of the one-piece cage, and the second tool half is provided with a plurality of second protrusions fixedly connected with the second tool half and extending into the cavity of the tool during the injection molding of the one-piece cage. Each of the first and second protrusions form a portion of a receiver pocket which is adapted to receive one of the rolling elements. The first and second protrusions are configured to form holding projections associated with each receiver pocket and integrated with the cage for holding the rolling elements in the cage.
The injection molding tool is embodied in such a way that a linear bearing cage can be produced by injection molding having guide faces as well as holding projections or protrusions for the individual rolling elements which do not project out of the basic geometry of the cage such as is the case with the construction described above in German Offenlegungsschrift 36 35 261. Moreover, the protrusions on the tool halves work together in such a way that they form receiver pockets for receiving the rolling bodies in the cooperating position and in the process also define the required holding protrusions for the rolling bodies.
An alternative version of the injection molding tool which is suitable for linear bearing cages which are provided with rolling elements in the form of rollers is designed to produce a one-piece cage, with the one set of protrusions being fixedly connected with one tool half and the other set of protrusions being fixedly connected with the other tool half. The protrusions on the tool halves are shaped in such a way that, when acting together, they form holding protrusions or elements for the rollers in the cage when the injection molding tool is closed, with such holding protrusions being integrated into the basic geometry of the cage. The protrusions on the tool halves are divided from one another, on the surface on which they lie next to each other when the injection molding tool is closed, at an angle of greater than zero degrees with respect to the closing direction of the injection molding tool.
In accordance with this aspect of the invention, the protrusions are in contact at a defined contact angle on the surface with which they come into contact when the injection molding tool is closed. As a result, the spherical embodiment of the holding protrusions, as well as of the contact surface of the rolling body with the cage, is in a divided form, by way of which it becomes possible to create an injection molding tool having advantages over other known techniques.
The protrusions can be formed without an undercut so that the cage can be freely unmolded after the injection molding process. In this way, a simple unmolding of the finished injection-molded cage can be performed without having to rely on the elasticity of the cage material. The injection molding tool is advantageously designed as a multiple mold, so that at least two cages can be produced per injection molding cycle.
The exact shaping of the cage geometry inside the receiver pockets for the rolling bodies is made easier in that each protrusion has a first section for forming a holding protrusion, a second section for forming a rolling body section, and a third section for forming the remaining portion of the receiver pocket. The protrusions are preferably formed so that they each respectively constitute one-half of a receiver pocket for the rolling elements.
A considerable simplification of the production of the injection molding tools can be realized by designing the tool so that the protrusions for forming the receiver pockets for rolling elements which lie one behind the other in the longitudinal direction of the cage are formed by one rail supported by one tool half. It is preferable that the respective rails of the two tool halves are designed to be identical to one another or geometrically equal. It is also possible to form each tool half with at least two rails for forming the geometry for a cage with at least two rows.
With this aspect of the invention, it is advantageously possible to create a precisely operating injection molding tool in a simple way. The protrusions which are needed for forming the receiver pockets for the rolling bodies are arranged on the rail, with the rail being very easily processed by way of finishing techniques. For example, it is possible to relatively exactly produce the contours of the protrusions by die erosion, and the lateral surfaces of the rails or the contact faces of the protrusions, by grinding. The rail finished in this way can then be inserted into the respective tool half of the injection molding tool and fixed in place. If two protrusions respectively define one half of the pocket geometry, it becomes possible in a very advantageous manner to divide the finished rails by a longitudinal cut and then fasten them on the tool halves. With multiple-row cages (for example double row cages), a finished rail can be divided into four equal parts which then, working together, are fastened on the halves of the injection molding tool. Through appropriate axial offset of the rails, it is possible in a very simple manner to produce the injection molding tool for producing cross roller cages.
A rolling bearing cage for a linear bearing produced in accordance with the present invention is in the form of a one-piece rolling bearing cage provided with a plurality of receiver pockets spaced from one another along a longitudinal extent of the cage. A rolling element is positioned in each of the receiver pockets, with the receiver pockets being provided with holding projections that hold the rolling elements within the receiver pockets of the cage and prevent the rolling elements from falling out of the receiver pockets. The holding projections are integrated into the cage to constitute an integral portion of the cage.
A rolling bearing cage for a linear bearing produced in accordance with another aspect of the present invention is in the form of a one-piece rolling bearing cage provided with a plurality of receiver pockets spaced from one another along a longitudinal extent of the cage. A rolling body is positioned in each of the receiver pockets, and the receiver pockets are provided with holding projections that retain the rolling bodies within the receiver pockets of the cage and prevent the rolling bodies from falling out of the receiver pockets. The holding projections are formed integrally with the cage. The receiver pockets are disposed in a first area of the cage that has a width at least 50% of the diameter of the rolling bodies, and the width in the other areas of the cage is less.
By virtue of the present invention, xe2x80x9cfreezingxe2x80x9d of the thermoplastic material during injection molding is not as likely to occur and is made more difficult because the cage body is made quite thick and solid. Accordingly, the thermoplastic material does not solidify as quickly as with other known cages. The width of the cage in the area of the receiver pockets is preferably at least 60% of the diameter of the rolling elements, and the cage in all areas other than in the area of the receiver pockets possesses a width that is at most 90% of the width of the cage in the area of the receiver pockets.
The holding projections for retaining the rolling elements do not project out of the cage, and are integrated into the areas of the receiver pockets of the cage. This is particularly useful when the length of the cage is relatively long, on the order of at least thirty times the diameter of the rolling elements.
The material to form the cage is material that can be injection molded, preferably injection molded thermoplastic material. The rolling bearing cage can be designed in such a way that at least two parallel rows of receiver pockets for receiving rolling elements are arranged in the cage to produce a so-called multi-row bearing.
The injection molding tool in accordance with the present invention makes it possible to produce a linear bearing cage in a particularly simple and therefore cost-effective manner, while still assuring that the cage geometry is relatively perfectly designed. This is particularly so with respect to the receiver pockets for the rolling bodies, i.e. the contact surface of the rolling bodies with the cage, and with respect to the holding projections which prevent the rolling bodies from falling out of the cage.
The injection molding process can be carried out in a very stable, and therefore reproducible manner, because the xe2x80x9cfreezingxe2x80x9d of the injected material is much less likely to occur. It is thus possible to relatively perfectly injection-mold linear bearing cages which are very long in comparison to their width.